No it cannot be true. The cel is : Check Engine LIGHT
Its just a light. Has no sensors attached to it.
If I were you I'd ditch the mechanic the instant he told me there is nothing wrong with your car just because at that very moment it seemed absolutely fine to him.
Well I've pointed out, or tried to point out the areas where the problem may be. If it were my car, I'd tell the mechanic, ORDER him to put a new relay in its place. It doesn't cost much to try that out. I'd rather do that than rip out all sorts of wires and change all sorts of expensive parts.
Its either a loose grip of the ecu, or the relay. Whatever is there in at the area you opened up today, you should check it out. If the relay box is installed in engine compartment and not in the area you guys operated on today, then you should first make sure that the ecu's grip is not loose and is not damaged. Do a careful visual examination.